1. Technical Field
This invention relates to an optical head for optically reading information which is magnetically recorded on a medium, and more particularly to an optical head using a polarization-maintaining optical fiber in an optical system.
2. Background Art
As an information medium of the type mentioned above, there is known a magneto-optic disk capable of recording, reproduction and erasure of information. Reproduction of magneto-optical signals is carried out by detection of the "Kerr effect", that is, the phenomenon observed at the time of reflection of a linearly polarized light by a magnetic recording film in which the light is reflected with a rotation of the plane of polarization of the light according to the direction of magnetization of the recording film at the point of reflection. There are two types of detection system for the Kerr effect: extinction type &lt;1&gt;, and differential type &lt;2&gt;.
The extinction type &lt;1&gt; of detection system will be explained referring to FIG. 9 of the accompanying drawings.
Laser light from a laser light source (not shown) is linearly polarized by a polarizer 1. The linearly polarized light is projected onto a magnetic recording film 2b provided on the back side of a substrate 2a of a magneto-optic disk 2 by coating. The light thus projected on the magnetic recording film 2b is reflected with rotation of the plane of polarization according to the direction of perpendicular magnetization of the recording film, by the Kerr effect. Where the principal direction of the polarizer 3 is set so that an extinction condition is obtained when the recorded information is "0", as shown in FIG. 10, the reflected light is transmitted through an analyzer 3 when the recorded information is "1". It is thus possible to read the magneto-optically recorded information through conversion of the information into electric signals by a detector 4 The intensity, Isig, of the light transmitted through the analyzer 3 when the recorded information is "1" is EQU Isig K.multidot.Io sin.theta..sup.2 (2.multidot..theta.k) (1)
where
Io: intensity of linearly polarized incident light, PA1 .theta.k: Kerr rotation angle, PA1 K: proportionality factor determined by PA1 .theta.k/2: the Kerr rotation angle of a magnetic recording film of the recording medium, PA1 n=0, 1, 2, . . . .
reflectance of recording film.
The representation in FIG. 10 is not based on the intensity of light but rather is based on the amplitude of light.
Now the differentiatype &lt;2&gt; of detection system will be explained below, with reference to FIG. 11.
Laser light from a laser light source (not shown) is linearly polarized by a polarizer 1, and the linearly polarized light is reflected by a magnetic recording film 2b provided on the back side of a substrate 2a of a magneto-optic disk 2 by coating. Where the principal direction of a polarization beam splitter (PBS) 5 is inclined by 45.degree., as shown in FIG. 12, the light component Is projected on axis S (s-polarized light component) is incident on a detector 4, while the light component Ip projected on axis P (p-polarized light component) is incident on a detector 4b. The intensities of the polarized light components Ip and Is, for a signal "1", are EQU Ip Io K/2(1+sin 2.theta.k) (2) EQU Is Io K/2(1-sin2.theta.k) (3).
The optical signals are received by the detectors 4a and 4b for photoelectric conversion to obtain electric signals, which are inputted to a differential amplifier 6 to pick up a difference signal. The quantity of the thus obtained signal, as estimated in the same manner as in the extinction type &lt;1&gt; above, is ##EQU1##
The magneto-optical recording films used at present have a Kerr rotation angle .theta.k in the range of .theta.k.ltoreq.1.degree.. Therefore, the extinction type &lt;1&gt; of detection system finds only limited use, because of the very small signal quantity [.infin. sin.sup.2 (2.theta.k)]. On the other hand, the differential type &lt;2&gt; of detection system provides a greater signal quantity [.infin. sin (2.theta.k)] and, therefore, enables signal detection with good S/N. Accordingly, the detection of magneto-optical signals is generally performed by the differential type &lt;2&gt; of detection system.
The construction of a magneto-optical head in general use at present will now be explained referring to FIG. 13. This is an example of the differentiatype &lt;2&gt; of detection system.
Light emerging from a semiconductor laser 11 is converted by a collimating lens 12 into parallel rays of light, which are subjected to linear polarization by a polarizing prism 13. The parallel rays of linearly polarized light are focused by a focusing lens 14 into a minute spot on a magnetic recording film 2b of a magneto-optic disk 2. The reflected light from the magnetic recording film 2b is branched by the polarizing prism 13 into two portions. The plane of polarization of one of the two branched portions of reflected light is rotated by 45.degree. by a halfwave plate 16, resulting in an azimuth shift of 45.degree. between a polarization beam splitter 17 and the polarizing prism 13. The p-polarized and s-polarized light components, which are respectively transmitted through and reflected by the beam splitter 17, are received respectively by detectors 18a and 18b, and subjected to photoelectric conversion. Electric signals output from the detectors 18a and 18b are input to a differential amplifier 19 to obtain a difference signal, whereby it is possible to detect the Kerr rotation angle .theta.k of the recording film and, hence, to reproduce the recorded information.
In order to shorten the period of time required for reproduction of desired information, i.e. the so-called access time, in a magneto-optic disk device and to achieve reductions in the size and power consumption of the device, it is essentially required to reduce the weight of an optical head itself.
In the integral-type construction as shown in FIG. 13, however, there are limits to the reductions in size and weight and it is impossible to realize adaptation to increasing operating speeds of the device.
In view of this, a separate-type head has been put to practical use, in which, as shown in FIG. 14, the head is separated into a stationary portion 20 and a movable portion 30, with a focusing lens 14 as a separate unit, and light is transferred between the portions 20 and 30 by spatial propagation.
Where the light is thus transferred by spatial propagation, however, a beam of light in a forward path and a corresponding beam of light in a backward path might discord from each other because of errors in positioning the rails for the movable portion (pitching, yawing and rolling). Thus, there would arise the problems as follows:
(i) aberration is generated in the focused beam, making it impossible to focus a light beam into a minute spot and, therefore, to perform high-density recording; PA0 (ii) deviations are generated in auto-focusing control or tracking control, necessitating correction means for such control;
and so on.
In view of the above problems, several proposals have been made concerning systems in which a stationary portion and a movable portion of an optical head are connected by optical fiber. However, none of the systems according to these proposals are satisfactory for practical use.